PROJECT SUMMARY The Surveillance, Epidemiology, and End Results (SEER) Program of the National Cancer Institute serves as a national dashboard for cancer surveillance and is an authoritative resource for cancer incidence and mortality rates. However, the registry lacks information on cancer recurrence, which is a major outcome impacting decisions and policies around management of disease at both the individual and the population levels. The goal of the proposed study is to develop a scalable approach for population-based ascertainment of cancer recurrence with a specific focus on breast cancer. The first (UG3) phase of the study will develop an innovative algorithm for detecting a first recurrence by combining medical claims with patient reported outcomes (PROs). This phase will be conducted in collaboration with the Seattle-Puget Sound SEER registry and will leverage ongoing research agreements with Puget Sound payers. Different algorithms will be developed to allow for flexibility in maximizing sensitivity or specificity. The algorithms will be validated against gold-standard recurrence information based on electronic medical records from two major local providers. Study participants will consistent of stage I-III breast cancer patients diagnosed in 2013?2016 on whom ?gold standard? information on recurrence is available. A retrospective case-control design will be employed in which an equal number of recurrent cases and non-recurrent controls at each validation site will be solicited for PROs regarding their recurrence histories. All validation activities will explicitly adjust for missing data resulting from participant non-response to the PRO solicitation. The second (UH3) phase will be conducted in collaboration with the Kentucky Cancer Registry. The algorithms developed in the UG3 phase will be deployed on all Kentucky stage I-III breast cancer diagnoses in 2015?2017 on whom claims data are available. This phase will include a validation against medical records from patients diagnosed at the University of Kentucky Markey Cancer Center over a concurrent period. In this phase, the study team will develop a visual interface for summarizing recurrence risks overall and within specified prognostic subgroups. The interface will allow users to examine representativeness of the patient cohort used to estimate recurrence risks and will also provide estimates that are adjusted for participant non-response. The proposed work will represent a pioneering effort that establish whether medical claims data combined with patient self-report constitute a promising avenue towards determining whether the critical endpoint of cancer recurrence can be added to population-based cancer registries.